The hiccup appears to be the result of a common carrier infrastructure glitch. T-Mobile told Engadget this was a "third party vendor issue that also affected other networks," while US Cellular told 92 Moose that a flawed update to the "cross carrier messaging system" was to blame. Sprint, meanwhile, pinned the issue on a "maintenance update." We've reached out to AT&T and Verizon (Engadget's parent company) as well. Carriers haven't gone into detail about what went wrong, though, so it's not clear why the texts resurfaced or what prevented at least some of them from being delivered in the first place.
T-Mobile and Sprint both said the issue has been fixed, so you shouldn't have to worry about more of these mystery texts. However, the issue was serious enough to have created some confusion. One user said she received a text from her now ex-boyfriend, while another got a message from a best friend that had passed away later in February. It's a not-so-subtle reminder that even relatively innocuous glitches like this can have real-world consequences.